Dual tone multifrequency tone generators are well known as shown by U.S. Pat. No. 3,184,554 to L. Meacham et al issued May 18, 1965. In this patent, tones are generated by resonant circuits coupled electromagnetically to a transistor amplifier.
More recently, tone signals have been generated digitally and synthesized into the actual tones for transmission. For example, U.S. Pat. No. 3,787,836 issued Jan. 22, 1974 to Hagelbarger shows one circuit for generating tones for telephone signaling while U.S. Pat. No. 4,061,886 to Callahan et al. Dec. 6, 1977 shows another. The latter of these patents shows a method of muting the telephone instrument transmitter and receiver during the duration of the tone signals emitted by the generator. As a first step, as the transmitter is switched off, a muting series resistor bleeds the capacitive charge from the switch element to produce a sharp cutoff of the transmitter. A muting resistor is switched in series with the receiver to mute the receiver against transmitted dial tones.
Other commercial tone generators such as that sold by American Microsystems, Inc. under the Model No. S2859 provide muting logic to stagger the shut off periods of the receiver and transmitter and to produce tone output only when both receiver and transmitter are muted. In such a system, once the key button is depressed, the receiver is muted first to prevent clicks or other noises from reaching the ear of the instrument user. Thereafter, the transmitter is muted to minimize the possibility of false tones being sent. With both transmitter and receiver muted, the output tones are emitted. When the button is released, the tone output is terminated and the transmit mute is released. After a timed period sufficient to dissipate any inductive charge in the system, the receiver muting is released.
When such systems are used with inductive hybrids, it has been found that inductively induced clicks may be generated to the receiver especially after the release of the tone generating key. The series muting alone is not sufficient to switch off the noise current resulting in an audible unpleasant sound transmitted to the ear of a user of the instrument.
In some cases, shunt muting is provided, the muting continuing for the duration of button duration and tone generation. This form of muting acts to load the